1. Field of the Invention
This invention relates to photometric devices of the TTL (through the lens) type, and more particularly, to an exposure meter in a single-lens reflex camera which measures the light rays passing through the reflex mirror of the camera.
2. Description of the Prior Art
A photometric device of the so-called TTL type exposure meter which measures light rays passing through the camera lens of a single-lens reflex camera wherein the light receiving element is arranged behind the reflex mirror to receive the light rays passing through the reflex mirror which is designed to partially pass the light rays coming from the camera lens and to reflex most of the light rays to the view finder optical system, has been known (Japanese Laid Open Specification No. 67623/1974). The camera provided with such a photometric device, however, has several disadvantages. Thus, as the light receiving element of the photometric device is arranged out of the path of the light rays passing through the reflex mirror, and the light transmitted to the reflex mirror is reflected to the light receiving element by a further reflecting member placed in front of the focal plane shutter, the reflecting member should necessarily be obliquely disposed with respect to the optical axis of the photo-taking lens. The reflecting member of the prior art is required to be large enough, especially in cameras equipped with the so-called center-weighted measurement system, to cover a wide photometric range; and, moreover, it becomes larger because the reflecting member is angularly disposed with respect to the optical axis. Therefore, the increased mass or inertial moment of the enlarged reflecting member may cause undesired movement or vibration when the reflex member is withdrawn from the photo-taking light path at the time of photographing.
Further, the angular disposition of the reflecting member may cause another difficulty in that a high degree of tolerance is required in the disposition of geometry of the reflecting member to maintain sufficient photometric accuracy, since a small difference in the disposition, or geometric error of the reflecting member greatly affects the advancing direction of the light reflected by the reflecting member towards the light receiving element.